Posting an annotated schematic showing exactly how you have wired it helps us help you. By showing all connections, power, ground and power sources you will save a lot of clarifying questions and time for all of us. Be sure to note any logic wires over 10/25 inches/cm in length. Post links to technical information on the hardware items including the motors, shield, and Arduino. This should include all component values or model numbers and details of all power supplies used (which could be USB power for example).
Posting the code following forum guidelines using code tags will also help. With this information we should be able to answer your question accurately. "How to get the best from this forum". How to get the best out of this forum